linux-kernel - Re: [PATCH] mm/swap: take folio refcount after testing the LRU flag

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Message-ID: <8e987e3f-6899-41d1-9ba4-c49cb91f7815@redhat.com>
Date: Mon, 12 Aug 2024 11:10:02 +0200
From: David Hildenbrand <david@...hat.com>
To: yangge1116@....com, akpm@...ux-foundation.org
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, 21cnbao@...il.com,
 baolin.wang@...ux.alibaba.com, liuzixing@...on.cn
Subject: Re: [PATCH] mm/swap: take folio refcount after testing the LRU flag

On 10.08.24 08:15, yangge1116@....com wrote:
> From: yangge <yangge1116@....com>
> 
> Whoever passes a folio to __folio_batch_add_and_move() must hold
> a reference, otherwise something else would already be messed up.
> If the folio is referenced, it will not be freed elsewhere, so we
> can safely clear the folio's lru flag. As discussed with David
> in [1], we should take the reference after testing the LRU flag,
> not before.
> 
> Link: https://lore.kernel.org/lkml/d41865b4-d6fa-49ba-890a-921eefad27dd@redhat.com/ [1]
> Signed-off-by: yangge <yangge1116@....com>
> ---
>   mm/swap.c | 8 +++-----
>   1 file changed, 3 insertions(+), 5 deletions(-)
> 
> diff --git a/mm/swap.c b/mm/swap.c
> index 67a2467..6b83898 100644
> --- a/mm/swap.c
> +++ b/mm/swap.c
> @@ -226,12 +226,10 @@ static void __folio_batch_add_and_move(struct folio_batch __percpu *fbatch,
>   {
>   	unsigned long flags;
>   
> -	folio_get(folio);
> -
> -	if (on_lru && !folio_test_clear_lru(folio)) {
> -		folio_put(folio);
> +	if (on_lru && !folio_test_clear_lru(folio))
>   		return;
> -	}
> +
> +	folio_get(folio);
>   
>   	if (disable_irq)
>   		local_lock_irqsave(&cpu_fbatches.lock_irq, flags);

Ah, "mm/swap: remove boilerplate" reduced them to a single instance, good.

You can throw in a

VM_WARN_ON_ONCE(!folio_ref_count(folio));

before the folio_get(folio)

so we would realize when something very unexpected happens.

Acked-by: David Hildenbrand <david@...hat.com>

-- 
Cheers,

David / dhildenb